Electro-osmotic pump

ABSTRACT

A pump for inducing liquid movement by means of polarisation electro-osmosis, comprising a passageway forming a flow path for fluid transport, at least one polarisable means located within said passageway so as to form at least one pore through said passageway, the at least one polarisable means being shaped such that at least a section of the pore walls are curved or inclined with respect to the longitudinal axis of the passageway, the pump further comprising a non-conductive porous membrane positioned across the flow path and in close proximity to the at least one polarisable means, the membrane comprising pores extending in a direction at least partially parallel to the longitudinal axis of the passageway and having a pore size smaller than the at least one pore formed by the polarisable means.

This invention relates to an improved electro-osmotic pump which usespolarisation electro-osmosis, for example electro-osmosis of the secondkind, to generate fluid movement, particularly within the field ofmicrofluidics.

Electro-osmosis is a well known phenomenon and is used in many differentfields. It relates to the motion of polar liquid through a porousstructure under the influence of an applied electric field. Mostsurfaces possess a negative charge due to surface ionisation. When anionic fluid is placed in contact with the surface, a layer of cationsbuilds up near the surface to screen this negative charge and maintainthe charge balance. This creates an electric double layer (EDL). When anelectric field is applied across the surface, the ions in the EDL areattracted towards the oppositely charged electrode, dragging thesurrounding medium with them due to viscous forces. This causes thefluid to move towards the negatively charged electrode.

Therefore, electro-osmosis can be used to control the movement of fluid.This has particular benefits in the field of microfluidics. Microfluidicstructures, or microsystems, consist of a series of microchannels andreservoirs, at least one dimension of which is generally in the micro-or nano-meter range and not greater than 1-2 mm. Fluids can be directedthrough these microchannels and subjected to a variety of actions suchas mixing, screening, detection, separation, reaction etc. Suchmicrostructures are of growing importance in chemical and biotechnicalfields as they allow tests and analysis to be carried out on a verysmall scale, thus reducing the amount of sample and reagents consumed ineach operation. This means work can be carried out quickly and at lessexpense than previously, with the production of fewer waste materials.Such microsystems are often referred to as “lab-on-a-chip”, orMicro-Total-Analysis Systems (μTAS).

The use of microfluidic actuators which utilise electro-osmosis isconsidered a promising technology for many microsystem applications, asthese actuators are relatively simple to fabricate and a goodperformance can be obtained for a wide range of ionic concentrations.

However, the above described form of electro-osmosis, known as classicalor ordinary electro-osmosis (EO1), normally requires a direct electricfield component to be present in order to obtain directed liquidtransport. This can result in several side effects, such as gasevolution at the electrodes and the establishment of pore concentrationprofiles along the pore axis, which reduces the efficiency andreliability of the system. These side effects can be reduced, althoughnot eradicated, by using a pulsed current.

Several other forms of electro-osmosis exist which do not require adirect electric field component to be present in order to operate. Thesephenomena are referred to herein collectively as “polarisationelectro-osmosis” and include induced charge electro-osmosis (ICEO) andelectro-osmosis of the second kind (EO2). Polarisation electro-osmosiscan be driven by either an alternating or direct current. Unlikeclassical electro-osmosis, in which the charge (the EDL) is alreadypresent and is simply set in motion by the application of an electricfield, in polarisation electro-osmosis the electric field also inducesthe charge which is then set in motion.

Induced charge electro-osmosis results from the action of an electricfield on its own induced diffuse charge near a polarisable surface.US2003/0164296 describes the use of ICEO to drive microfluidic pumps andmixers. This phenomenon is sometimes referred to as AC electro-osmosis(ACEO), especially in situations in which ICEO is used to pump liquidsusing flat asymmetric electrodes and an AC voltage.

Electro-osmosis of the second kind acts on ions within a space chargeregion (SCR) associated with the surface. Transport by EO2 is 10-100times faster than for classical electro-osmosis at the same electricfield strength. Consequently, fast transport of liquid can be achievedat relatively low potentials. Microfluidic pumps utilisingelectro-osmosis of the second kind are described in WO2004/007348.

In both of these types of polarisation electro-osmosis, the surface usedto generate the diffuse charge layer or SCR is provided by one or morepolarisable elements, usually spherical particles. The surface must bepolarisable in order to create a build up of the charge imbalancenecessary for the electric field to act upon. When these polarisableelements are immersed in electrolyte fluid (the fluid to be transported)and subjected to a suitable electric field, the necessary diffuse chargeor SCR will be generated and the fluid will move via polarisationelectro-osmosis. For ICEO, symmetrical flow patterns will be obtained oneach side of a spherical particle, resulting in zero net flow, which isuseful for mixing purposes. ICEO can be used for directed pumping byshielding one side of the spherical particle, or by adapting the shapeof the polarisable element in order to create a greater charge imbalanceon one side. In EO2 the flow is generated only or mainly at one side ofthe particle.

The velocity of the flow generated by polarisation electro-osmosis isproportional to the size of the polarisable elements used. When placedwithin a microchannel, or other fluid passageway, these polarisableelements reduce the cross sectional flow area through the passageway andeffectively create pores, formed by the spaces between the polarisableelements, through which fluid can flow. The size of polarisable elementsrequired in order to generate sufficient flow speeds creates arelatively large pore size and hence limits the pumping pressure of thedevice.

For example, when creating a EO2 pump it is necessary to use polarisableelements with a characteristic diameter, D_(char), of at least 10 μm.The characteristic diameter is defined as the diameter of thepolarisable element(s) measured parallel to the direction of theelectric field applied during operation.

This results in a trade off between the benefits of polarisationelectro-osmosis (which increases with the size of the polarisableelements) and the pump pressure.

One option to mitigate this trade off is to use polarisable elements inthe shape of cigars or cylinders. These still provide a curved surface,which can induce polarisation electro-osmosis, but can be packed closertogether, hence reducing the pore size within the passageway. However,such polarisable elements are difficult to produce, especially at thesmall sizes which would be required to generate a high pressure in manymicrofluidic applications. For example, often the pore size should bebelow one micrometer.

Viewed from a first aspect the present invention provides a pump forinducing liquid movement by means of polarisation electro-osmosis,comprising

a passageway forming a flow path for fluid transport,

at least one polarisable means located within said passageway so as toform at least one pore through said passageway,

the pump further comprising

a non-conductive porous membrane positioned across the flow path and inclose proximity to the at least one polarisable means, the membranecomprising pores extending in a direction at least partially parallel tothe longitudinal axis of the passageway and having a pore size smallerthan the at least one pore formed by the polarisable means, whereby, inuse, an electric field generated across the polarisable means in thelongitudinal direction of the passageway will cause fluid in thepassageway to flow under the action of polarisation electro-osmosis. theat least one polarisable means being shaped such that at least a sectionof the pore walls are curved or inclined with respect to thelongitudinal axis of the passageway,

Therefore, the invention lies in the realisation that existingpolarisation electro-osmosis pumps can be improved by the addition of aporous membrane acting in combination with the polarisable means of thepump.

All polarisation electro-osmotic pumps require polarisable means asthese provide the surfaces at which the charge imbalances necessary toinduce movement by polarisation electro-osmosis, such as the diffusecharge layer or SCR, can form. Placing a porous membrane in closeproximity to the polarisable means allows the generated charge to extendinto the pores of the membrane. This occurs because the charge flowparallel to the membrane surface is inhibited by the porous structure ofthe membrane, and hence the charge layer grows thicker. In other words,lateral drift of the induced charge is reduced by the porous membrane.

Increasing the thickness of the generated charge layer increases itseffect. Therefore, when a porous membrane is used in accordance with thepresent invention, both the polarisable means and this membrane cangenerate polarisation electro-osmotic flow. Flow can be increased for agiven electric field.

In order to achieve this the porous membrane must be in close proximityto the polarisable means, as the induced charge layer which forms aroundthis means is very thin. Therefore, the membrane should be no more than5 micrometres from the polarisable means.

The possibility of enhancing EO2 by extending a generated SCR into aporous structure was suggested in an article by Mishchuk et al,“Electroosmotic Transport of Fluid through a Diaphragm-Resin System”(Journal of Water Chemistry and Technology, Vol. 26, No. 4, pp. 21-32,2004). This article relates to the use of EO2 in electrofiltrationprocesses. These processes are used in order to, for example, extractimpurities from water and to remove heavy metals from soils. In suchprocesses a charged porous diaphragm can be used to induceelectro-osmotic movement through the diaphragm, leaving the unwantedparticles behind. Although highly theoretical, this article hypothesisesthat by coating the diaphragm in an ion exchange resin the SCR generatedby the resin granules will extend into the diaphragm, thus increasingthe electro-osmotic effect. It is suggested that flow through a chargedporous diaphragm could be enhanced by using this in combination with anion exchange resin or that an uncharged diaphragm could be used incombination with the resin to provide electrofiltration processes.

The inventors of the present invention have realised that this techniquecould also have advantages in relation to polarisation electro-osmosispumps. In addition, it has been appreciated that the use of a porousmembrane can offer further advantages.

The use of a porous membrane in accordance with the present inventionhas the further benefit of enabling the effective pore size of thepassageway to be reduced. By using a separate component to control thepore size of the pump, the polarisable means can be designed solely witha view to providing the desired level of polarisation electro-osmoticeffect. The polarisable means can therefore be made larger withoutsacrificing the pumping pressure, as the porous membrane can be used tokeep the pore size of the pump small.

The “pores” formed by the polarisable means can comprise pores orinterstices within the means itself but also the spaces formed betweenthe passageway walls and the polarisable means. In other words, thepores formed by the polarisable means are the areas through which fluidflowing through the passageway can pass by or through the polarisablemeans. Therefore it is not necessary for the polarisable means tocontain pores itself. Instead this can act to partially block thepassageway such that the remaining spaces act as pores.

In accordance with the present invention, the porous membrane mustcontain pores of a smaller size than the pores formed by the polarisablemeans. The pore size of the pores formed by the polarisable means is,for the purposes of the present invention, taken to be the same as thepore size of a membrane with straight cylindrical pores having the samelength and porosity as the pores provided by the polarisable means, andwith the same void volume to surface area ratio. Such a calculation ofequivalent pore sizes is frequently done when considering fluid flow,see W. L. McCabe, J. C. Smith, and P Harriot. Unit operations ofchemical engineering. McGraw-Hill, Singapore, 1993, page 152-153.

The membrane is preferably positioned across the entire flow path suchthat it would block the passageway if it were not porous. This ensuresthat the whole of the flow path is affected by the pore size of themembrane.

It is not necessary for all of the pores of the membrane to be the samesize, or indeed the same orientation. It is possible for example, forthe porous membrane to comprise a first set of pores extending in afirst direction and a second set of pores extending in a seconddirection which is perpendicular to the first. Alternatively themembrane may comprise a first set of pores having one size and a secondset of pores having a different size. Preferably however the membranecomprises uniform pores which are unidirectional.

It is necessary for at least some of the pores of the membrane to extendin a direction at least partially parallel to the longitudinal axis ofthe passageway, such that fluid flowing through these pores will exitthe membrane at a point longitudinally remote from the point at which itentered the membrane.

Preferably the pores of the membrane are substantially parallel to thelongitudinal axis of the passageway.

The pore size of the membrane will be determined in part by the functionof the pump. In situations in which a very low flow rate is required butat high pressure, pores as small as 30 nm could be used. On the otherhand, when a higher flow rate is desired, for example for electroniccooling, larger pores, e.g. up to 10 μm would be beneficial.

Within the field of microfluidic systems, it has been found that the useof membranes having a pore size of between 0.1-1 μm produces anoticeable increase in pumping ability.

However, more generally the pores of the porous membrane are preferablyat least ten, most preferably at least one hundred, times smaller thanthe length of the polarisable means when measured in a directionparallel to the longitudinal axis of the passageway.

Ideally, the thickness of the membrane should be of a similar magnitudeto the enhanced diffuse charge layer, SCR or other charge concentrationthat will be induced. This ensures that all of the extended diffusecharge layer, SCR etc is utilised without introducing any additionallength which would simply add resistance to the system.

However, finding this preferred thickness in practice would be timeconsuming and difficult. A membrane thickness of 100 μm has producedgood results in microfluidic systems. More generally, a membranethickness of between 10 and 1000 μm is preferred, more preferablybetween 50 and 200 μm.

A single membrane may be used, or plural membranes may be used. It maybe desirable to use more than one membrane, for example in order toachieve the desired overall membrane thickness. Where plural membranesare used they will normally be in face to face contact. The membranethickness as referred to herein means the overall thickness, i.e. thethickness of a single membrane or the combined thickness of pluralmembranes. In certain embodiments two membranes are used.

The membrane is non-conductive as the use of a conductive membrane wouldshort circuit the electro-osmotic effect.

Although the membrane can have the opposite or no surface charge, it ispreferable for the membrane to have a surface charge of the same sign,and preferably the same surface groups, as the polarisable means.Different charges will lead to a braking effect on the electro-osmoticflow, whereas using two different surface groups might lead to undesiredelectrochemical reactions.

Preferably the membrane comprises a hydrophobic polymer. This preventsthe occurrence of a soaked membrane matrix, which would not contributesignificantly to liquid transport, while leading to increased currentand power consumption. This type of membrane also has the advantage thatit does not retain liquid after the liquid within the pores is removedby liquid transport.

Preferably the hydrophobic membrane material is selected from polymermaterials such as polypropylene, polytetrafluoroethylene (PTFE) or(ultra high molecular weight) polyethylene. Preferably the hydrophobicmembrane is treated during manufacture with a hydrophilising surfacetreatment. This treatment could, for example, introduce sulfonic acidgroups, onto the pore surface. As mentioned above, these groups arepreferably the same as the surface groups of the polarisable means.

As mentioned previously, the porous membrane must be placed in closeproximity to the polarisable means so that the induced charge imbalancegenerated on the surface of the polarisable means can extend into themembrane. In order to achieve the best results it is preferable that theporous membrane is in direct contact with the polarisable means.

The polarisable means can be in any form suitable for inducingpolarisation electro-osmotic movement. Such means are known from priorart polarisation electro-osmotic pumps.

In certain preferred embodiments, the at least one polarisable means isshaped such that at least a section of the pore walls are curved orinclined with respect to the longitudinal axis of the passageway. Thusthe pores formed by the polarisable means may have walls which arecurved or inclined with respect to the generated electric field, whichis typically parallel to the longitudinal axis of the passageway in thevicinity of the polarisable means. The field within the pores will thenhave both normal and tangential components, thus the normal electricfield component induces a charge on the polarisable surface, and thetangential component sets the charge in motion resulting in liquidmovement along the surface. Thus polarisation electro-osmotic flow isachieved.

In these embodiments, it is not necessary for the entire pore wall to becurved or inclined. In some cases the passageway may have straight sidesand these may form one or more sides of a pore. However each pore shouldpreferably contain at least a section of pore wall, formed by thepolarisable means, which is curved or inclined with respect to thelongitudinal axis of the passageway. Although the passageway may, in itsentirety, contain bends and changes in direction, the polarisable meanswill generally be positioned in a straight segment of the passagewayalong which an electric field can be applied. Therefore all reference tothe longitudinal axis or direction of the passageway refers to thesection of passageway containing the polarisable means.

The at least one polarisable means may have a portion having a surfacein close proximity to the porous membrane and substantiallyperpendicular to the longitudinal direction of the passageway. Under theeffect of an electric field a charge will be generated on this surfaceand so a generated charge layer is formed. The generated charge layerwill tend to extend into the pores of the membrane and across the poresof the polarisable means, with the result that flow will be generated.The polarisable means of such embodiments may additionally have curvedor inclined walls, but these are not necessary. Therefore, in certainembodiments the at least one polarisable means has only surfaces whichare substantially perpendicular to the longitudinal direction of thepassageway and surfaces which are substantially parallel to thelongitudinal direction of the passageway. An example is a polarisablemeans with straight cylindrical pores arranged parallel to thelongitudinal direction of the passageway. The polarisable means may be amembrane with such pores, i.e. a second membrane.

For the embodiments having a surface in close proximity to the porousmembrane and substantially perpendicular to the longitudinal directionof the passageway, the relationships between the thickness and pore sizeof the porous membrane and the polarisable means are the same as thosedescribed above.

In one embodiment the polarisable means comprises at least onepolarisable particle. When placed in the passageway this restricts theflow and hence forms a pore. This pore has the same length as theparticle as measured in the longitudinal direction of the passageway.The addition of more particles can change the shape of the pore orcreate a plurality of pores.

Preferably the polarisable particle is substantially spherical, as theseare relatively simple to manufacture. However, many other shapes arepossible, for example, cylindrical, oval, parallelogram, triangular,kite-shaped, frusto-conical etc.

When individual polarisable particles are placed in contact with oneanother a single charge is produced and the electro-osmotic effect isincreased. Therefore preferably the polarisable means comprises aplurality of adjoining polarisable particles.

The adjoining polarisable particles may be arranged in rows or gridswithin the passageway, or simply be randomly packed or clusteredtogether. Increasing the number of particles positioned “side by side”in a plane substantially perpendicular to the longitudinal axis of thepassageway increases the number of pores while increasing the thicknessof the polarisable means by adding particles in the longitudinaldirection increases the pore length. Having adjoining layers ofpolarisable particles adds to the strength of the generated polarisationelectro-osmotic effect but only up to a certain threshold, whereupon theaddition of further layers will not affect the flow rate obtained.

In embodiments in which the polarisable means comprises a plurality ofadjoining polarisable particles, it is preferred that the porousmembrane is in close proximity to or contacts more than one polarisableparticle. For example, when the polarisable particles are arranged inthe same plane across the passageway it is preferable for the porousmembrane to contact each polarisable particle, i.e. the entire outersurface of the polarisable means.

It is also possible in some embodiments for a single porous membrane tocontact, or to be in close proximity to, more than one polarisablemeans. This can occur when two or more polarisable particles areattached to the wall of the passageway at radially spaced intervals inthe same cross sectional plane. Together these polarisable meansrestrict the flow path and create a pore.

When the polarisable means comprises one or more polarisable particlesit is preferable that the pores of the porous membrane are at least tentimes smaller than the length of a single polarisable particle asmeasured in the longitudinal direction of the passageway. Mostpreferably the pore size is at least one hundred times, smaller thanthis length.

Preferably the porous membrane has a thickness of between 0.5 and 3times the size of one polarisable particle measured in the longitudinaldirection of the passageway.

Alternatively, the polarisable means may be provided in the form of apolarisable membrane (in some embodiments having pores which are curvedor inclined with respect to the longitudinal axis of the passageway, orin other embodiments having straight cylindrical pores). In this case,the non-conductive membrane pore size should preferably be at least tenand more preferably at least 100 times smaller than the thickness of themembrane constituting the polarisable means as measured in thelongitudinal direction of the passageway.

As mentioned previously, electro-osmosis of the second kind (EO2)provides fast flow rates at relatively low voltages compared toclassical electro-osmosis. Therefore, in a preferred embodiment the pumpis arranged to induce liquid movement by electro-osmosis of the secondkind.

In order to achieve this, it must be possible to generate an SCR at thesurface of the polarisable means. This is accomplished by using aunipolar conducting material. So that the generated SCR can be used toprovide directed movement of the fluid the surface of the unipolarconducting means should be uniform. By uniform it is meant that anysurface irregularities must be less than 5% of the characteristicdiameter of the polarisable means (the characteristic diameter being thedimension of the polarisable means measured parallel to the direction ofthe generated electric field). The polarisable means may compriseregular structures (such as grooves) in the flow direction. However,preferably the polarisable means has a smooth surface, i.e. all surfacediscrepancies are less than 5% of the characteristic diameter.Preferably any surface discrepancies are less than 100 nm, regardless ofdiameter.

Although it is possible to construct the present invention with anyunipolar conducting means, preferably the polarisable means should haveone or more of the following properties: an ionic conductor, made frompolymer, have strongly acidic ion exchange groups (e.g. sulfonic acidgroups), conductivity 10 or more times that of the fluid medium, bulkconductivity (the member could have a non-polarisable core, but thediameter of this should preferably be significantly smaller that that ofthe conductive outer layer, more preferably zero), lowest possibleporosity and pore-size (ideally zero). In particular it is preferablethat the unipolar conducting means comprises a strongly acidicion-exchanger. Sulfonated polystyrene-divinylbenzene is a particularlypreferred material, although sulfonated acrylic could also be used,especially for applications where reduced protein binding is beneficial.When the polarisable conducting means is a membrane with conical pores,sulfonated polyether ether ketone (sPEEK) mixed with polyethersulfone(PES) is a preferred material. Such a membrane can be produced bycasting the dissolved polymer mixture onto a surface having protrusionsrepresenting the negative of the pore shape. This surface can be made ofpolydimetylsiloxane (PDMS) replicated from an etched silicon master.

Metal, semiconductor, metallic polymers and other ion-exchangers couldbe used for special applications, but are generally less preferred.

By unipolar conductor is meant a material for which the conductivity ishigher for either negative or for positive charges, preferably at least4 times higher, more preferably at least 15 times higher.

When the pump is arranged to operate via ICEO the polarisable means cancomprise a dielectric material. However, preferably a conductingmaterial is used. Therefore, in both EO2 and ICEO pumps the polarisablemeans is preferably a conducting means.

To ensure a suitably high conductivity ratio between the conductingmeans and the fluids likely to be used within the pump it is preferablefor the conducting means to have a specific conductivity of greater than0.01 S/cm, more preferably between 0.01-1S/cm and most preferablygreater than 1 S/cm.

When the polarisable means comprises a conducting means, the porousmembrane can be considered to be non conductive if it has a specificconductance at least 5 times lower than the conducting means.

When the pump is arranged to operate via EO2, it is preferable for theporous membrane to be provided such that it is in close proximity to orcontacts the unipolar conducting means on the surface on which, in use,the SCR is generated. However, in addition it has been surprisinglyshown that placing the membrane on the opposing side of the polarisablemeans, or placing porous membranes on both sides, also enhances thepumping effect.

This benefit is also found in pumps which operate using other forms ofpolarisation electro-osmosis. Therefore, in one embodiment, the pumpfurther comprises a second porous membrane in close proximity to,preferably in contact with, the opposing surface of the polarisablemeans to the porous membrane, the second porous membrane having poreswhich extend in a direction at least partially parallel to thelongitudinal direction of the passageway. Preferably the pores of thesecond porous membrane are larger than the pores formed by thepolarisable means. This lowers the flow resistance created by thisadditional membrane.

The use of a second porous membrane can serve the additional purpose offixedly securing polarisable particle(s) in place within the passagewaywhen these are used to form the polarisable means. This is advantageousas it negates the need for a bonding agent or precision engineering tohold the polarisation means in place.

It is not necessary for a second porous membrane to be used for thisfunction. Instead it is also possible for the polarisable means to becontained between the porous membrane and another porous structure, forexample an open grid. Therefore, preferably the polarisable means isfixedly located within the passageway between the porous membrane and aporous sealing element, such as a second membrane, net or grid.Preferably the pores of the sealing element are larger, most preferablyat least 3 times larger, than the pores formed by the porous membrane.

A further option is for the porous membrane to be provided in the formof a porous plug, which encases the polarisable means. This has theadvantage that it would be easier to manufacture, although would produceless electro-osmotic effect.

Alternatively, the porous membrane may comprise a porous film over thesurface of the polarisable means, which can be applied throughconventional spraying techniques.

As mentioned above, when polarisable particles are used, increasing thenumber of adjoining particles only results in a corresponding increasein polarisation electro-osmotic effect up to a certain threshold.Preferably therefore a plurality of polarisable means are providedspaced apart in the longitudinal direction of the passageway. In thisway, the pump can, in effect, contain a number of separate pumpingsections. The plurality of polarisable means can each be individualparticles, adjoining particles, membranes or a combination thereof.

When a plurality of polarisable means are used, it is preferred that thepump further comprises a plurality of porous membranes, such that eachpolarisable means is in close proximity to, or in contact with, a porousmembrane having pores at least partially parallel to the longitudinalaxis of the passageway and having a pore size smaller than the poresformed by the polarisable means.

This allows each polarisable means to enhance the polarisationelectro-osmotic flow by extending the charge concentration, e.g. theSCR, into the pores of a membrane.

In certain preferred embodiments, the pump comprises a pair ofnon-conductive porous membranes, one on each side of the at least onepolarisable means and each in close proximity to the at least onepolarisable means. With such an arrangement the pump is bidirectional.

In some embodiments, each polarisable means is further in contact with aporous sealing element on the opposing surface to the porous membrane soas to fixedly locate the polarisable means. This sealing element couldbe a second porous membrane, grid, net etc.

All of the membranes used within the pump may have the same thickness,pore sizes and material properties as discussed above. Thus where asecond porous membrane is used it can serve both the mechanical functionof locating the polarisable means, as well as enabling the pump to beoperated bidirectionally.

It is possible for the porous membrane of a second, downstreampolarisable means to be in contact with a first, upstream polarisablemeans. In other words, the longitudinally spaced polarisable means canbe separated only by a porous membrane which contacts both groups. Insuch embodiments the porous membrane itself acts as a sealing element.

Alternatively, the gap between longitudinally spaced polarisable meanscould be filled by a sealing element having a pore size much greater,e.g. at least 5 times, preferably 10 times the pore size of the porousmembrane. This sealing element may comprise e.g. a second porousmembrane or grid.

In embodiments in which the membrane comprises a porous plug, this plugcan enclose one or more polarisable means or separate plugs can be usedin respect of each longitudinally spaced polarisable means within thepump.

The pump can be produced without electrodes. However, these must bepresent in use in order to produce an electric field across thepolarisable means. These can be added to the pump once this is in placewithin an operating system but could also be an integral part of thepump. Therefore, preferably, the pump further comprises electrodespositioned on either side of the polarisable means in the longitudinaldirection of the passageway which, in use, are arranged to generate anelectric field across the polarisable means so as to cause fluid to flowunder the action of polarisation electro-osmosis.

The electrodes can encompass a plurality of longitudinally spacedpolarisable means or alternatively each longitudinally spacedpolarisable means may have its own pair of electrodes. Each electrodemay be spaced from a respective non-conductive porous membrane or it maybe in direct contact therewith so as to cover its surface. Good resultscan be achieved with the electrode in contact with the porous membrane.In embodiments where a single porous membrane is provided in closeproximity to the polarisable means, the electrode on that side of thepolarisable means may be in contact with the porous membrane, whilst inembodiments where a pair of porous membranes are provided, one on eachside of the polarisable means, an electrode on one side of thepolarisable means may be in contact with the porous membrane on thatside and the electrode on the other side of the polarisable means may bein contact with the porous membrane on that other side.

Palladium is a preferred material for the electrodes.

Preferably the electrodes comprise a surface area directed towards thepolarisable means. Such electrodes could be porous metal grids parallelto and covering (part of or the entire) passageway cross section. Theycould also be three-dimensional structures, such as pillars, extendingfrom the channel walls. Such electrodes will provide a more even currentdistribution. This is particularly important when using EO2 in order toensure an SCR is generated in all parts of the polarisable means.

Alternatively, the electrodes can comprise a metal or conducting polymerlayer deposited onto the porous membrane and/or sealing element oneither side of the polarisable means. For example, gold or other metalscould be deposited by sputtering or evaporation, and conducting polymerscould be deposited by coating with a dissolved polymer. The conductingmaterial should be deposited in a controlled way in thin layers, so theyform thin conducting layers on top of the membranes and sealingelements, rather than rendering the bulk of the sealing elements andmembranes conducting. This form of electrode is beneficial when thepolarisable means are positioned between membranes or other sealingelements such as grids. Making use of the existing structure of the pumpto form electrodes in this manner is cost effective.

Therefore, preferably the porous membrane(s) and sealing element(s)comprise an electrode layer deposited on their outer surface.

The electric signals required to generate polarisation electro-osmoticmovement are known in the art.

Due to the larger induced charge which is built up when using a porousmembrane in close proximity to the polarisable means, liquid velocitieswill typically be higher than the velocities obtained in a pump whichdoes not comprise a porous membrane at a given field strength. Thevelocity of an EO2 pump can be increased by up to ten times and thepressure can increase by more than 10 times. It is therefore possible toreduce the voltage used to operate the pump. This brings severaladvantages, including reduced problems with electrochemical reactionsand gas formation, simpler control electronics and power supply andincreased portability.

According to theory, during the short period of charge build-up in anEO2 pump in accordance with the present invention, the liquid velocity(v) is expected to depend on the electric field strength E, size ofconductive means a, membrane pore size a_(m) and bulk ionicconcentration C as follows:

${ v \sim E^{\frac{5}{3}}}a^{\frac{2}{3}}a_{m}^{2}C_{0}^{\frac{2}{3}}$

However, after charge build-up, the following relation is assumed tohold for the case of an inert (i.e. non-conductive) membrane withstraight cylindrical pores parallel to the direction of the electricfield:

${ v \sim E^{\frac{2}{3}}}a^{\frac{2}{3}}a_{m}^{2}C_{0}$

This means that the overall pump shows a sub-linear velocity-electricfield strength relation for this extreme case (i.e. pores parallel tothe direction of the electric field). Most porous materials will haveporosity also in the direction perpendicular to the electric field,allowing for flow of liquid and charge in this direction, although stillinhibited.

Although the exact flow-voltage relation can vary with the porosity andpossibly other properties of the inert porous material, the entering ofthe charge into this more finely porous material and the concomitantthickening of the charged layer will lead to a large increase in flowrate and pressure compared to the situation without such inert porousmaterial.

The pump can be operated using an asymmetric square pulse signal with anoffset or zero DC component, as is frequently used for EO2 and ICEO. Forsystems with a sub-linear relationship between pressure/flow and theelectric field, the weak pulse will determine the flow direction,whereas for a super-linear relation the strong pulse determines thedirection.

For asymmetric pumps (with a finely porous membrane only at one side ofthe polarisable means) symmetrical AC signals can be used to obtaindirected transport. DC signals can also be used.

As the use of a porous membrane increases the electro-osmotic effect,the electrodes can be positioned further from the polarisable means thanin previous pumps. In some embodiments this may be beneficial as theelectrodes can create unwanted side effects.

In certain embodiments, therefore, a spacer layer is provided betweenthe porous membrane and the electrodes in order to reduce electricalresistance and unwanted bubble formation. Preferably, this spacer is inthe form of an ion exchange membrane, which allows the electrical chargeto be transmitted to the polarisable means but prevents any bubbles frompassing. It is further possible to provide polarisable means in contactwith this ion exchange membrane. This will produce an electro-osmoticconvection effect and reduce the undesired polarisation on the spacermembranes (which would lead to increased electrical resistance and hencethe need for higher voltages). However, for dilute ionic solutions (upto 0.1 to 1 millimolar) the lower voltage and the possibility to use ACfor the present invention will usually make this measure unnecessary.

The pump of the present invention can be provided within a microchannelof a microfluidic system. In such embodiments the passageway of the pumppreferably forms part of a microchannel. The electrodes for providingthe electric field can be positioned within the microchannel on eitherside of the polarisable means, thus defining a pumping segment withinthe channel. Multiple pumps could be placed in the same system or eventhe same microchannel, as desired.

Therefore, viewed from a further aspect the present invention provides amicrofluidic system comprising:

a microchannel;

electrodes positioned within said microchannel, defining between them apumping segment;

at least one polarisable means located within said pumping segment as soto form at least one pore through said pumping segment;

a non-conductive porous membrane positioned across the pumping segmentand in close proximity to the at least one polarisable means, themembrane comprising pores extending in a direction at least partiallyparallel to the longitudinal axis of the pumping segment and having apore size smaller than the at least one pore formed by the polarisablemeans; and

the pumping segment being arranged such that, in use, the electrodesgenerate an electric field across the polarisable means so as to causefluid in the microchannel to flow under the action of polarisationelectro-osmosis.

In certain preferred embodiments, the at least one polarisable means isshaped such that at least a section of the pore walls are curved orinclined with respect to the longitudinal axis of the pumping segment.Alternatively the at least one polarisable means may have only surfaceswhich are substantially perpendicular to the longitudinal direction ofthe passageway and surfaces which are substantially parallel to thelongitudinal direction of the passageway.

However, it is also possible for the pump of the present invention to befree standing. This allows the pump to be manufactured and soldindependently so that this can then be used in a variety ofapplications. For example, a free standing pump could be connected tothe inflow conduit of a microfluidic “lab-on-a-chip” system. In thisway, the pump could still be used to pump fluid through a microsystemwithout the complexity of incorporating this in to the microsystemitself.

In such embodiments the passageway is preferably a through hole within ahousing. The polarisable means preferably comprises a plurality ofadjoining polarisable particles packed within the through hole betweensaid porous membrane and a porous sealing element. In this way theplurality of polarisable particles are contained within the throughhole. In a preferred embodiment the sealing element is a second membranehaving pores which extend in a direction at least partially parallel tothe longitudinal direction of the passageway. It is preferred that thepores of the second membrane are larger than the pores formed by thepolarisable means. Alternatively the sealing element may comprise a gridor net.

The use of a free standing pump is considered inventive in its own rightand therefore, viewed from a further aspect the present inventioncomprises a pump for inducing liquid movement by means of polarisationelectro-osmosis, comprising a housing containing a through hole therein,said through hole forming a passageway; porous sealing elements sealingat least a section of the through hole; a plurality of adjoiningpolarisable particles packed between said sealing elements, thepolarisable particles creating pores through said passageway; whereinone of said sealing elements comprises a non-conductive porous membranecomprising pores extending in a direction at least partially parallel tothe longitudinal axis of the passageway and having a pore size smallerthan the pores created by the polarisable particles.

The polarisable particles may be shaped such that at least a section ofthe pore walls are curved or inclined with respect to the longitudinalaxis of the through hole. Alternatively they may have only surfaceswhich are substantially perpendicular to the longitudinal direction ofthe passageway and surfaces which are substantially parallel to thelongitudinal direction of the passageway.

The second sealing element may comprise a further membrane or a grid asdescribed above.

The free standing pump may also comprise electrodes positioned on eitherside of the polarisable particles. These can be located outside thethrough hole or alternatively the electrodes may be positioned withinthe through hole on either side of the polarisable particles. This couldbe achieved by creating a through hole, filling this with polarisableparticles, sealing the through hole using porous membranes and thenextending this through hole by the addition of annular rings on eitherside of the housing.

Several embodiments of the invention shall now be described, by way ofexample only, with reference to the accompanying figures, in which:

FIG. 1A shows a cross section through a microchannel containing a priorart pump;

FIG. 1B shows a cross section along line B-B of FIG. 1A;

FIG. 2A shows a pump according to the present invention;

FIG. 2B shows another pump according to the present invention

FIG. 3A shows the principle of EO2 on a single polarisable particle;

FIG. 3B shows the principle of EO2 on a single polarisable particle incontact with a porous membrane;

FIG. 4A shows another version of the pump according to the presentinvention;

FIG. 4B shows another version of the pump according to the presentinvention;

FIG. 4C shows another version of the pump according to the presentinvention;

FIG. 5 shows a further version of the present invention;

FIG. 6 shows a further embodiment of the present invention;

FIG. 7 shows an embodiment of the present invention in which the pump isfree standing;

FIG. 8 shows a further embodiment of the present invention;

FIG. 9 shows a schematic representation of a porous polarisable membranefor use in the present invention;

FIG. 10 shows another embodiment of the pump of the present invention;and

FIG. 11 shows an enlargement of the part show as “X” in FIG. 10.

FIG. 1A shows a prior art pump 10 within a microchannel 12. This pump 10is arranged to operate via electro-osmosis of the second kind.Electrodes 13 a, b extend into microchannel 12 and define a pumpingsegment 14 within the microchannel 12. Within this pumping segment 14 anumber of spherical polarisable particles 15 are arranged to form apolarisable means 11. These particles 15 are made of a unipolarconducting material and are curved with respect to the longitudinal axisof the microchannel 12 and the electric field provided by electrodes 13a, b. When the microchannel is filled with an electrolyte, such asmethanol, and electrodes 13 a, b are operated to provide a suitableelectric field across the polarisable means 11, this induces movement ofthe surrounding fluid via electro-osmosis of the second kind. Therefore,this pump allows fluid to flow through the microchannel 12 in thedirection indicated by the arrows.

A cross section through microchannel 12 is shown in FIG. 1B. This crosssection is taken along line B-B, i.e. through the polarisable means 11.As can be seen, the polarisable means 11 partially blocks themicrochannel 12 and forms pores 17 through the channel. The shape of thepolarisable particles 15 is such that they provide the pores 17 withcurved walls. In this instance the microchannel 12 itself also forms acurved pore wall, however this is unnecessary and in other embodimentsthe microchannel may be square or rectangular in cross section. Thecharge imbalance required to generate polarisation electro-osmoticmovement will be induced at the surface of the polarisable means 11 andthis which creates a curved or inclined pore wall. Due to the requiredsize of the polarisable particles 15 the pores 17 created between theseare relatively large, which limits the pump pressure.

The pump of the present invention, an embodiment of which is shown inFIG. 2A, overcomes this problem. The pump 20 includes a porous membrane26 positioned across the microchannel 22 such that it contacts thepolarisable means 21, which consists of spherical polarisable particles25. The membrane 26 comprises narrow pores 26 a which are parallel tothe longitudinal axis of the microchannel 22. Apart from this addition,pump 20 comprises identical components to prior art pump 10.

A variation is shown in FIG. 2B. In this embodiment, the electrode 23 ais in contact with the porous membrane 26. The electrode 23 a may be alaser perforated foil, or a mesh or grid, of metal or other conductivematerial.

The porous membrane 26 affects the pump 20 in two beneficial ways.Firstly, it reduces the effective pore size of the pump 20, as themembrane is selected such that its pores 26 a are narrower than thepores formed by the polarisable means 21. This allows the pumpingpressure to be increased. Secondly the electro-osmotic effect isincreased. This is due to the effect that the membrane 26 has on thespace charge region (SCR) generated around the polarisable means 21 uponapplication of a suitable electric field. The principles of SCR areexplained below with reference to FIGS. 3A and B.

An SCR is induced on the polarisable means 21 if the generated electricfield is strong enough to give a strong concentration polarisation. Thepolarisation zone then consists of a diffusion zone at the boundary withthe bulk liquid, an SCR layer closer to the polarisable surface, andpossibly an EDL closest to the surface. However, despite the possibilityof an EDL at the polarisable particle surface it is important to notethat the SCR is established independently of this layer. The notion of“electro-osmosis of the second kind” indicates the similarity to EO1 byhaving its source in a thin charged zone, the SCR, which is differentfrom electric effects working on the bulk liquid (electro-hydrodynamiceffects). Such polarisation phenomena have been described for bothionically and electronically conducting materials.

The polarisation phenomenon can be described most simply with referenceto a permselective (cat)ion conducting material in some liquid of lowerconductivity. This phenomenon is well known, and will be describedbriefly here. By directing an electric field towards the material,cations are transported towards and through the solid material, while noanions are allowed to pass in the opposite direction, due topermselectivity (i.e. the transport number of one charge issignificantly larger than that of the other). At a steady state, theelectro-diffusional flux of co-ions away from the material iscompensated by a diffusional flux in the opposite direction. Thus, adiffusion zone with concentration decreasing towards the material isobserved. Upon increasing the electric field strength, the currentincreases while the concentration decrease becomes larger. A limit isreached at zero ion concentration near the material. At this point, nocurrent increase is observed upon further increasing the voltage, thusthe term “limiting current”.

However, while the limiting current represents a plateau in thevoltage-current curve, a further increase in current takes place if thevoltage is high enough. One feature of this strong concentrationpolarisation is the appearance of the SCR close to the material (betweenthe material and the diffusion zone).

One reason for the appearance of over limiting current is the appearanceof EO2 eddies (circular flows, sometimes referred to aselectroconvection) in the polarisation zone, adding to diffusional iontransport. Even at a flat membrane, EO2 eddies are observed.

FIG. 3A shows the SCR produced on a single unipolar conducting particlein some liquid of lower conductivity and in a strong electric field,indicated by arrow E. The curved surface of the particle creates twocomponents to the main electric field, tangential component E_(tan) andnormal component E_(norm). The normal component induces a thin SCRlayer, while the tangential component acting on this layer results inion and liquid transport-electro-osmosis of the second kind. As shown inthis figure, the SCR generated is very thin. However, when multiplepolarisable particles are grouped together a single, combined SCR isproduced.

The SCR can be further increased by the provision of a porous membrane36 contacting the particle. As shown in FIG. 3B, the SCR extends intothe pores 36 a of the membrane. This occurs as the free space betweenthe particle and membrane is not large enough to allow for unrestrictedelectro-osmotic flow. Therefore, the polarisation region thickens andextends into the membrane. A larger layer of SCR produces a larger EO2effect.

This explanation relates to EO2. However, weaker (not involving overlimiting current) polarisation phenomena such as induced charge electroosmosis (ICEO) can be enhanced in the same way.

Consequently a pump 20, having a porous membrane 26 will produce agreater electro-osmotic effect and in addition the reduced effectivepore size of the pump 20 will increase the pumping pressure. This allowselectrodes 23 a, b to be positioned further from the polarisable means21, thus increasing the stability of the pump 20.

Due to the unipolar nature of the polarisable means 21, an SCR onlybuilds up on one side of the particles. Therefore the membrane 26 isusually placed on the side of the polarisable means 21 on which, in use,the SCR will be induced. However, benefits can also be obtained byplacing the membrane 26 on the opposing side of the polarisable means,or having a membrane on each side.

FIG. 4A shows an embodiment of the pump 40 in which a further membrane47 is included on the opposing side of the polarisable means 41. Thismembrane 47 acts as a sealing element which, together with porousmembrane 46, fixedly locates the polarisable means 41 within thepassageway 40. Membrane 47 has larger pores in order to reduce thehydrodynamic resistance. Membrane 46 is positioned on the side of thepolarisable means 41 on which, in use, the SCR is generated. In analternative version of this embodiment, it is possible for electrodes 43a, 43 b to be formed by a layer of metal or conducting polymer depositedon the membranes 46, 47.

In a variation shown in FIG. 4B, a pair of membranes 46 with fine poresis provided, one on each side of the polarisable means 41. Thisarrangement provides a bidirectional pump. Although the pump involves agreater hydrodynamic resistance than the pump of FIG. 4A, due to thepresence of a finely porous membrane on the downstream side of thepolarisable means 41, it has the advantage that it can be operated ineither direction.

In an alternative version, shown in FIG. 4C, the electrodes 43 a and 43b are provided each in contact with a respective membrane 46. Eachelectrode 43 a, 43 b may be a laser perforated foil, or a mesh or grid,of metal or other conductive material.

FIG. 5 shows a further embodiment in which pump 50 has two polarisablemeans 51 a, b comprising a plurality of polarisable particles 55 forgenerating electro-osmotic movement. Both of these means 51 a, b are incontact with a separate porous membrane 56 a, b positioned so as toenhance the generated SCR.

It is also possible for the polarisable means 51 a, b to be positionedcloser together, or for membrane 56 b to be thicker, so that thismembrane 56 b also contacts the first polarisable means 51 a. Furtherpolarisable means 51 could also be included as desired.

The embodiment shown in FIG. 5 further includes an alternativepassageway configuration in which electrodes 53 a, b are situated incompartments 54 off the main passageway 52. Flow through the passageway52 by-passes these compartments 54, as indicated by the arrows. Ionexchange membranes 58 are located close to the electrodes 53 a, b andseparate the compartments 54 from the main passageway 52. These ionexchange membranes 58 do not block the electric field but they doprevent bubbles from reaching the polarisable means 51 a, b. Suchbubbles can form when, for example, a direct current is used to powerthe pump 50. The compartments 54 could hold a lower pressure than thepassageway 52. For example, they could be open to the atmosphere orcontain a lid making it possible to exchange (buffer) solution and letout electrolytic gases.

FIG. 6 shows another embodiment of the invention. Here the pump 60comprises a plurality of polarisable means 61 a, b, each being contactedby a porous membrane 66 a, b and a second sealing membrane 67 a, b sothat the same advantages as discussed in relation to FIG. 4 areachieved. Compartments 64 and ion exchange membranes 68 are again usedto prevent bubbles from interfering with the operation of the pump 60.In this embodiment however additional polarisable means 69 are placed incontact with these membranes 68. These additional polarisable means 69create electro-osmotic convection which reduces the undesiredpolarisation effects of the ion exchange membranes 68.

The above embodiments all relate to pumps formed within microchannels.While the pump of the present invention is particularly suited to use inmicrosystems, it can also be used in many other fields.

FIG. 7 shows a free standing pump 70 in accordance with the presentinvention. This pump can be attached to many different fluid systems,such as electronic cooling circuitry, or a feeder inlet to amicrosystem. In this latter application the pump 70 can still be used tooperate a microsystem without the complexity of integrating this withinthe system itself.

Pump 70 is created within housing 71. A passageway 72 is drilled throughthis housing 71 and tightly packed with polarisable particles 75 suchthat every particle is in contact with another particle, thus forming asingle polarisable means. This ensures that a single, combined SCR isgenerated during use. Having multiple layers of polarisable particles 75increases the electro-osmotic effect up to a certain depth, after whichincreasing the number of layers will not have any further effect on thegenerated SCR. Pump 70 has an excess number of layers, meaning that notall of these will further the electro-osmotic effect, however creatingthe pump 70 on this scale eases production.

The passageway 72 is sealed at each end by porous membranes 76, 77.These can be identical or the membrane which in use will not carry anyof the generated SCR can have larger pores in order to reduce theresistance of the pump 70. In an alternative embodiment a grid could beused to seal this end of the passageway.

Spacer layers 78 are coated over the housing 71 and then electrodes 73a, b are attached to the outer side of these layers 78 by any suitablemeans.

Pump 70 can then be attached to flow channels as required.

Although the above embodiments have used spherical polarisableparticles, other polarisable means are also possible. In some cases theyalso comprise a surface with is curved or inclined to the generatedelectric field.

FIG. 8 shows a pump 80 in which the polarisable means 81 is in the formof a hollow cylinder having a tapering inner surface such that afrustoconical pore 87 is formed. The tapered inner surface provides aninclined surface for producing an SCR. Porous membrane 86 is placed incontact with the polarisable means 81 in order to reduce the effectivepore size of the pumping segment and to enhance the electro-osmoticeffect.

Alternatively the pore 87 of FIG. 8 could be formed by a number ofseparate polarisable wedge-shaped particles radially spaced about thepassageway wall. In such an embodiment each wedge would form apolarisable means.

In other embodiments the polarisable means can be a porous membrane,having pores of a shape similar to polarisable means 81. FIG. 9 shows aschematic representation of a suitable pore shape for a polarisablemembrane 90. Here, pore 91 comprises a sloped, inclined section 92 and astraight walled section 93. Therefore in this embodiment only a sectionof the pore wall is inclined with respect to the longitudinal axis ofthe passageway. Sloped section 92 is pyramidal in shape, while section93 has a square cross section. The sloping section 92 is thicker thenthe straight section 93 in order to provide a larger surface for SCRgeneration. An example of suitable dimensions for the membrane 90 are11=50 μm, 12=1-2 μm, w1=5 μm, w2=5 μm. The sloping section 92 can havean inclination of 54.7 degrees, and hence follow the crystal planes ofsilicon.

Such a membrane 90 could be used, for example, in textiles to provide awater transport function. In such embodiments, in line with theinvention, a porous membrane could be placed in contact with one or bothsides of the polarisable membrane.

It will be appreciated that the embodiments described above arepreferred embodiments only of the invention. Thus various changes couldbe made to the embodiments shown which would fall within the scope ofthe invention. For example, although the above embodiments have beendescribed with reference to the use of EO2, these could equally beadapted to provide movement via ICEO or any other form of polarisationelectro-osmosis.

Another preferred embodiment is described with reference to FIGS. 10 and11. This embodiment is similar to that of FIG. 2 except that instead ofusing spherical polarisable particles it uses a polarisable membrane 27.The pump 20 includes a non-conductive porous membrane 26 positionedacross a microchannel 22 such that it contacts the polarisable means,which consists of the polarisable membrane 27. Electrodes 23 a, 23 bextend into the microchannel 22.

The polarisable membrane 27 comprises straight cylindrical pores 28which are parallel to the longitudinal axis of the microchannel 22. Thepolarisable membrane 27 has surfaces 29 in close proximity to the porousmembrane 26 and which are perpendicular to the electric field. In fact,the polarisable membrane 27 and the porous membrane 26 are arranged indirect (face to face) contact and are shown slightly separated in FIG.11 for the purposes of illustration only.

The walls of the pores 28 of the membrane 27 are formed by surfaces 29 awhich are parallel to the direction of the electric field. Therefore, inthis embodiment, the polarisable membrane 27 has only surfaces 29 whichare substantially perpendicular to the longitudinal direction of thepassageway and surfaces 29 a which are substantially parallel to thelongitudinal direction of the passageway. There are no surfaces whichare curved or inclined with respect to the longitudinal direction.

The porous membrane 26 also comprises pores 26 a which are parallel tothe longitudinal axis of the microchannel 22, but these are narrowerthan the pores 28.

A charge layer is generated by an electric field on the surfaces 29which are substantially perpendicular to the longitudinal direction ofthe passageway and covered by the non-conductive porous membrane 26. Thegenerated charge layer will tend to extend into the pores 26 a of themembrane 26 and across the pores 28 of the polarisable membrane 27. Thecharge induced on the surfaces 29 will overlap and form a continuousspace charge region (SCR) extending fully or partly through thethickness of the non-conductive porous membrane 26. As the SCR extendsacross the pores 28 of the polarisable membrane 27, pressure and flow isgenerated. Therefore, in this embodiment, the combination of thepolarisable membrane and the non-conductive membrane leads to flow underthe action of polarisation electro-osmosis, without the polarisationmembrane having any surfaces which are inclined or curved with respectto the longitudinal direction of the passageway. The effect provided bythe combination of a polarisable membrane and a non-conductive membraneis also obtainable with other embodiments, and is not limited to thisparticular example.

1.-41. (canceled)
 42. A pump for inducing liquid movement by means ofpolarisation electro-osmosis, comprising a passageway forming a flowpath for fluid transport, at least one polarisable means located withinsaid passageway so as to form at least one pore through said passageway,the pump further comprising a non-conductive porous membrane positionedacross the flow path and in close proximity to the at least onepolarisable means, the membrane comprising pores extending in adirection at least partially parallel to the longitudinal axis of thepassageway and having a pore size smaller than the at least one poreformed by the polarisable means, whereby, in use, an electric fieldgenerated across the polarisable means in the longitudinal direction ofthe passageway will cause fluid in the passageway to flow under theaction of polarisation electro-osmosis.
 43. A pump as claimed in claim1, wherein the at least one polarisable means is shaped such that atleast a section of the pore walls is curved or inclined with respect tothe longitudinal axis of the passageway.
 44. A pump as claimed in claim1, wherein the at least one polarisable means has only surfaces whichare substantially perpendicular to the longitudinal direction of thepassageway and surfaces which are substantially parallel to thelongitudinal direction of the passageway.
 45. A pump as claimed in claim1, wherein the membrane comprises uniform pores which areunidirectional.
 46. A pump as claimed in claim 1, wherein the porousmembrane is in direct contact with the at least one polarisable means.47. A pump as claimed in claim 1, wherein the membrane has a pore sizeof between 30 nm and 10 μm.
 48. A pump as claimed in claim 1, whereinthe pores of the porous membrane are at least ten times smaller than thelength of the polarisable means when measured in a direction parallel tothe longitudinal axis of the passageway.
 49. A pump as claimed in claim1, wherein the membrane has a thickness of between 10 and 1000 μm
 50. Apump as claimed in claim 1, wherein the membrane has a surface charge ofthe same sign as the polarisable means.
 51. A pump as claimed in claim1, wherein the polarisable means comprises at least one polarisableparticle.
 52. A pump as claimed in claim 10, wherein the polarisablemeans comprises a plurality of adjoining polarisable particles.
 53. Apump as claimed in claim 10, wherein the porous membrane has a pore sizeat least 10 times smaller than the size of one polarisable particlemeasured in the longitudinal direction of the passageway.
 54. A pump asclaimed in claim 1, wherein the polarisable means comprises a porousmembrane.
 55. A pump as claimed in claim 1, wherein the polarisablemeans is fixedly located within the passageway between the porousmembrane and a porous sealing element having pores which extend in adirection at least partially parallel to the longitudinal axis of thepassageway.
 56. A pump as claimed in claim 1, comprising a pair ofnon-conductive porous membranes, one on each side of the at least onepolarisable means and each in close proximity to the at least onepolarisable means.
 57. A pump as claimed in claim 1, wherein the pumpfurther comprises electrodes positioned on either side of thepolarisable means in the longitudinal direction of the passageway which,in use, are arranged to generate an electric field across thepolarisable means so as to cause fluid to flow under the action ofpolarisation electro-osmosis.
 58. A pump as claimed in claim 16, whereinat least one of the electrodes is in direct contact with the porousmembrane.
 59. A pump as claimed in claim 1, wherein the passagewaycomprises a through hole within a housing, and the polarisable meanscomprises a plurality of adjoining polarisable particles packed withinthe through hole between said porous membrane and a porous sealingelement.
 60. A microfluidic system comprising: a microchannel;electrodes positioned within said microchannel, defining between them apumping segment; at least one polarisable means located within saidpumping segment as so to form at least one pore through said pumpingsegment; a non-conductive porous membrane positioned across the pumpingsegment and in close proximity to the at least one polarisable means,the membrane comprising pores extending in a direction at leastpartially parallel to the longitudinal axis of the pumping segment andhaving a pore size smaller than the at least one pore formed by thepolarisable means; and the pumping segment being arranged such that, inuse, the electrodes generate an electric field across the polarisablemeans so as to cause fluid in the microchannel to flow under the actionof polarisation electro-osmosis.
 61. A pump for inducing liquid movementby means of polarisation electro-osmosis, comprising: a housingcontaining a through hole therein, said through hole forming apassageway; porous sealing elements sealing at least a section of thethrough hole; a plurality of adjoining polarisable particles packedbetween said sealing elements, the polarisable particles creating poresthrough said passageway; wherein one of said sealing elements comprisesa non-conductive porous membrane comprising pores extending in adirection at least partially parallel to the longitudinal axis of thepassageway and having a pore size smaller than the pores created by thepolarisable particles.